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ULAS J222711-004547
Dominated by dust
Federico Marocco
A.C. Day-Jones1, H.R.A. Jones1, R.L. Smart2, Z.H. Zhang1, P.W. Lucas1, J.I. Gomes1,
B. Burningham1, D.J. Pinfield1, R. Raddi3, L. Smith1
1 University of Hertfordshire
2 Osservatorio Astrofisico di Torino
3 University of Warwick
Introduction
Faherty et al. 2013
Brown dwarfs do not form a “Main Sequence”.
Because of the mass-age-luminosity degeneracy objects of similar spectral type can have
very different physical properties (e.g. age, mass, metallicity, surface gravity)
The wide range of parameters covered is reflected by the NIR colours.
Introduction
The same effect can be seen in the spectra.
The huge differences in the
NIR spectra of these late Ls
are due to the large spread
in ages sampled, from the
very blue esdL7 to the very
red dL7.5.
However, as usual there are
exceptions...
Zhang et al. 2013
Age
Unusually red L dwarfs
There is infact a category of L dwarfs with very red
NIR colours and spectra, that do not appear to be
particularly young (e.g. Kirkpatrick et al. 2010).
Low gravity is not sufficient to explain their peculiar
colours (e.g. Allers & Liu 2013).
Gizis et al 2012
Looper et al 2008
ULAS J2227-0045
One of the most extreme URLs is ULAS J2227-0045, with MKO J-K= 2.79.
Candidates Selection
ULAS J2227-0045 comes out of a
Large sample of L and T dwarf
candidates, selected from the
UKIDSS LAS DR7
(Day-Jones et al. 2013).
Criteria:
- J <= 18.1
- Y-J >= 0.8
- z-J >= 2.4 and J-K >= 1.0
OR
z-J >= 2.9 and J-K < 1.0
- i-z > 2.0
- i-J > 4.7
- z-K > 3.5 and J-K < 1.0
Final candidates list: 263 objects,
to be followed-up with X-shooter(Day-Jones et al. 2013)
Spectroscopic follow-up
We obtained a optical+NIR spectrum using
VLT/Xshooter.
Very red spectrum, peaks in the H band.
We classify this object L7 via spectral fitting.
We split the spectrum in three parts (optical+J,
H and K band) and separately normalize them
to remove the “red excess”, then we fit it to the
spectroscopic standards (Burgasser et al. 2006;
Kirkpatrick et al. 2010).
When compared directly to the standard,
ULAS J2227-0045 show a massive H and
K band “excess”
Photometry
ULAS J2227-0045 is detected
in UKIDSS, WISE and
2MASS (H and K band only):
MKO J = 18.11
MKO J-K= 2.79
2MASS J-K= 3.04
J-W2= 4.47
W1-W2= 0.653
The object marks an extreme
in the L-T sequence.
The peculiarity is more
prominent in the NIR than
in MIR.
Spectroscopic follow-up
The spectrum looks very similar to other
low-gravity dwarfs and URLs.
They all show smooth optical spectra and
very red SEDs, peaking in the H band.
The spectral indices defined in Allers &
Liu 2013 do not point towards low gravity
for ULAS J2227-0045.
Astrometry
We have estimated the proper motion using 2MASS, UKIDSS and WISE
µαcosδ = 100 ± 16 mas/yr µδ = -30 ± 16 mas/yr
Not a member of any young moving group. So what causes the H and K band excess?
Dust?
Model fitting
Models do not reproduce the
spectrum properly.
H2O and CO absorption bands, and
the shape of the H band peak are
the major issues.
NB: models do not include
higher-than-solar metallicity
Ref:
Allard et al. 2011 (BT-Dusty & BT-Settl)
Madhusudhan et al. 2011 (A & AE)
Tsuji 2005 (UCM)
De-reddening
We have de-reddened the
spectrum of ULAS J2227-045
using the Fitzpatrick 1999
extinction curve.
The spectrum fits quite well the
L7 standard with an E(B-V) = 1.1.
The asymptotic reddening of the
field is 0.07, i.e. the reddening of
the spectrum is intrinsic.
De-reddening
The main dust species in L
dwarfs are thought to be
corundum (Al2O3), enstatite
(MgSiO3) and iron (e.g. Morley
et al. 2012).
We derived extinction curves
for these 3 species for a range
of grain sizes (r = 0.05 to 1.00
µm) and applied them to the
spectrum of ULAS J2227-0045.
Corundum and enstatite give
good fit for grain size of
0.40-0.55 µm.
De-reddening
De-reddening
Iron requires smaller grains but does not give good results in the optical part of the spectrum.
De-reddening
The de-reddening works for other URLs too.
The grain sizes obtained are consistent with
those obtained for ULAS J2227-0045.
When going to earlier types (e.g.
2MASS J0355+1133, L5ɣ) the quality of the
fit decreases.
De-reddening
You can push the de-reddening even further!
ULAS J2227-0045 de-reddened with enstatite
Summary & Conclusions
ULAS J2227-0045 is one of the reddest L-dwarfs known to date, and fits into the category
of URLs.
Spectral indices and kinematic show that its extreme colours are not caused by low
gravity.
An alternative explanation can be an excess of dust in the photosphere, maybe caused
by unusual metallicity.
De-reddening the spectrum of URLs and young objects using extinction curves for
different dust species gives surprisingly good results.
Dust opacity seems to be enough to explain the diversity of objects, at least in the
mid-to-late L regime.
Thank you!